When the intensity of the electric current exceeds, during a certain time, the rated intensity of an appliance, it is necessary to cut off the electrical supply in order to avoid any damage to that appliance by excessive overheating. A bimetal through which the electric current flows to deform it by heating, is currently used against long overcharges, to cause the cutting off of the electric current, the cut-off occurring more or less rapidly as a function of the intensity of the current. When that intensity is greater than the rated intensity, this protection is called thermal protection and comes into play for intensities generally not exceeding twice the rated intensity in the operating state.
In the case of instantaneous overcharges of current, a relay having a winding traversed by the electric current and an armature subjected to the action of a return spring are used; when a great overcharge of current occurs, even momentarily, the blade is attracted and the movement thereof controls the cutting off of the electricity supply; this type of protection is called magnetic protection and generally comes into play for intensities whose instantaneous value exceeds twice the rated intensity.
Thermal and magnetic protection are provided separately and are used more particularly in circuit-breakers; they are generally very bulky and difficult to adjust and their characteristics vary in time; moreover, the compensation of thermal devices on multi-pole circuit-breakers is generally not provided on each device, but is provided by a single compensating bimetal which then provides only an average and hence imperfect compensation.